This invention relates to a screw propeller with no shaft boss (hereinafter referred to as bossless screw propeller) designed to be mounted in the hull for effecting back and forth or rightward and leftward movements of the ship, and a ship thruster adapted with said bossless screw propeller and disposed at the bow and/or ship bottom to allow easy and quick movement of the ship for its coming alongside or leaving a pier.
A screw propeller mounted in the ship bottom close to the stern has been used for effecting the forward or backward movement of the ship, while the ship movement for bringing the ship alongside the pier or moving it away from the pier has been accomplished by ejecting water to the right or left side of the ship by the propellers of the thruster units mounted centrally in the holes formed at the bow and stern so that such holes are in communication sidewise with each other at the ship bottom.
However, since the conventional screw propellers are of the shaft propeller system and the same system is used for the thruster units as well, the water ejected out by the propellers produces a hollow screw thrust due to the presence of the shaft boss, thus resulting in a limited thrust output. Another serious problem of the conventional shaft propeller type thruster devices is that the shaft torque is necessarily increased, necessitating the use of a high-power engine.